1. Field of the Invention
The present invention relates to a logarithmic amplifier, and more particularly to a logarithmic amplifier designed to prevent an abnormal output even when an excessive input is received.
2. Description of the Prior Art
Current signals having a very wide range such as output signals from a photodetector which reads, for example, a high gradation image are often subjected to logarithmic amplification in order to make the current signals easy to handle by compressing the range. As disclosed in Unexamined Japanese Patent Publn. No. Hei-2(1990)-277181, a logarithmic amplifier for carrying out logarithmic amplification has been widely known which has an operational amplifier for logarithmic conversion purposes, and a feedback circuit for feeding back an output of the operational amplifier via a resistor connected to an output terminal of the operational amplifier.
However, as generally admitted, if such a conventional logarithmic amplifier has an input which is so large that it cause an output voltage of the operational amplifier to overshoot, the logarithmic amplifier temporarily cannot output a normal signal value, and this persists for a while even after the input has returned to a normal level. FIGS. 1A to 1C show the previously mentioned problem. In other words, if a current shown in FIG. 1A is input to the logarithmic amplifier, an output voltage shown in FIG. 1A will be obtained if the logarithmic amplifier properly works. However, in some cases, the output voltage often shows an abnormally low value for a period of t.sub.0 after the input has returned to its normal level, as shown in FIG. 1C.